In the context of gas turbines used for generating electricity, it is known that these can be operated not only at, but also below, their rated load. This mode, termed partial load operation, can however lead to a substantial excess of air during combustion of the fuel; the combustion air ratio is then substantially greater than 1. In that context, the lower the power to be provided by the gas turbine, the greater the air excess can be. In this case, the combustion temperature of the fuel-air mixture in the combustion chamber drops, which has an analogous effect on the primary zone temperature which is relevant for CO emissions. If this temperature then undershoots a minimum value, increased CO emissions are generated. If the primary zone temperature is further reduced, the CO emissions can rise to such a point that they exceed an emissions limit value—which is generally set by law—so as to depart from the partial load range of the gas turbine in accordance with CO emissions limits. This can result in the operator of the gas turbine being forced—if such a legal CO emissions limit value exists—to switch off their gas turbine, that is to say that it is possible to further reduce the power of their gas turbine and to simultaneously bring CO emissions below the limit value.
In order to further increase the partial load capability, just described, of the gas turbine, the prior art document DE 10 2008 044 442 A1 proposes equipping such gas turbines with a bypass system through which part of the compressor end air is guided past the combustion chamber and can be fed into the exhaust gas duct of the gas turbine. It is thus possible to reduce the quantity of air supplied to the combustion, which raises the combustion temperature and thus the relevant primary zone temperature. The raise then leads to a reduction of CO emissions, such that, in spite of further-reduced load operation, the gas turbine can be operated in a manner which confirms to CO emissions limits. A disadvantage is, however, that the mode of operation known in the prior art unnecessarily reduces the efficiency of the gas turbine, since the bypassed compressed air does not contribute to the work conversion in the gas turbine.